In many applications it is desirable to monitor the current flowing in an electronic component, and to compare it with a reference value to determine whether or not the electronic component is working in an overcurrent condition.
Most power management circuits such as AC-DC converters, DC-DC converters, battery management circuits, linear regulators and the like usually have internal overcurrent protection/detection circuits. Also motor driver circuits usually have internal overcurrent protection/detection circuits to prevent damage and/or malfunction.
Commonly, a voltage representative of the current flowing in an output electronic component or in a load (such as motors, batteries, converters load) is produced on a series sense resistor Rsense, as illustrated in FIGS. 1 and 2. Of course, the use of a resistor increases the power dissipation and decreases the overall system efficiency. Moreover, monitoring circuits for reading the voltage drop on the sense resistor need dedicated pins.
To reduce disadvantages, the architecture of FIG. 3 is often used. The power transistor supplying the current to the load is coupled with a scaled replica thereof (SenseFet) connected in parallel to the power transistor. Therefore, the current through the sense transistor SenseFet is a scaled replica of the current flowing through the power transistor PowerFet.
The replica current Isense may be compared with a reference current IREF to determine whether or not the power transistor is in an overcurrent condition. The circuit for comparing the replica current should not alter the bias condition of the transistor SenseFet with respect to that of the transistor PowerFet, otherwise the sensed replica current Isense may not be proportional to the current Ipower flowing through the Power Fet.
A typical way to ensure this is to use the architecture illustrated in FIG. 4, comprising a voltage comparator for comparing the voltage drop on a sense resistor in series to the SenseFet with a threshold V(IREF).
With this architecture, the result of the comparison is affected by fluctuations of the sense resistor, fluctuations of the V(IREF), comparator offset, and PowerFet-SenseFet mismatch.
Some of these variations can be compensated, for example, by obtaining the threshold V(IREF) with a voltage drop on a resistor of the same type of the one used as current sense resistor, as shown in FIG. 5. However, the result of the comparison remains affected by sense resistor—reference resistor mismatch, comparator offset, and PowerFet-SenseFet mismatch.